1. Field of the Invention
The present invention relates generally to an apparatus and method for performing uplink (UL) power control in a broadband wireless access (BWA) system. More particularly, the present invention relates to an apparatus and method for reducing a UL inter-carrier interference (ICI) in a BWA system.
2. Description of the Related Art
Extensive research is being conducted to provide various Quality of Service (QoS) features having a data rate of about 100 Mbps in the advanced fourth-generation (4G) communication system. The 4G communication system is evolving to provide mobility and high QoS in a BWA system such as a Local Area Network (LAN) system and a Metropolitan Area Network (MAN) system. The BWA system uses an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme.
In general, schemes for link adaptation to address signal attenuation depending upon a distance of a mobile station (MS) from a base station (BS) include a rate adaptation scheme and a power adaptation scheme. Depending on the scheme being used, an OFDMA system adjusts a data rate or a power level for uplink (UL) transmission in adaptation to the signal attenuation depending on the distance from a BS, thereby providing the desired link performance. Compared to the power adaptation scheme, the rate adaptation scheme provides a larger system capacity by changing a data transmission rate depending on the channel conditions of respective mobile stations (MSs).
FIG. 1 is a graph for comparing a rate adaptation scheme and a power adaptation scheme in terms of carrier-to-interference ratio (CIR), and FIG. 2 is a graph for comparing a rate adaptation scheme and a power adaptation scheme in terms of Tx power.
As illustrated in FIGS. 1 and 2, the rate adaptation scheme transmits signals from all MSs in a cell to a BS at the same power regardless of the distance from the BS and changes a data transmission rate in consideration of a CIR change depending on the distance from the BS. Unlike the rate adaptation scheme, the power adaptation scheme increases the Tx power of an MS by the amount of power attenuation depending on the distance from a BS, so that all the MSs in a cell can have the same CIR.
However, when a data rate scheme for UL transmission in the OFDMA system is adapted to address signal attenuation depending on the distance from a BS, a severe ICI may occur due to the difference of speed and Rx power between MSs.
FIG. 3 is a graph illustrating a severe ICI problem that occurs due to the difference of BS Rx signal power between an MS located at the center of a cell and an MS located at the boundary of the cell when all the MSs in the cell transmit UL signals at the same Tx power.
As illustrated in FIG. 3, a signal transmitted from the MS located at the cell center undergoes small attenuation and is thus received at high power in a BS, while a signal transmitted from the MS located at the cell boundary undergoes large attenuation and is thus received at low power in the BS. In this case, the signal transmitted from the MS located at the cell boundary acts as an ICI with the signal transmitted from the MS located at the cell center, which may cause performance degradation.
Therefore, in the uplink of the OFDMA system, the Tx power of an MS that has a large channel gain (i.e., small signal attenuation) and a high speed needs to be reduced in order to reduce a severe ICI caused by another MS.